a. Field of Invention
The present invention relates to an exhaust valve for an internal combustion engine. In particular it relates to a two-stroke crosshead engine, comprising a movable spindle with a valve disc of a nickel-based alloy which also constitutes an annular seat area at the upper surface of the valve disc, which seat area abuts a corresponding seat area on a stationary valve member in the closed position of the valve. The seat area of the valve disc has been subjected at its manufacture to a thermo-mechanical deformation process at which the material is at least partially cold-worked.
b. Description of Related Art
The development of exhaust valves for internal combustion engines has aimed for many years at extending the life and reliability of the valves. This has been done so far by manufacturing the valve spindles with a hot-corrosion-resistant material on the lower disc surface and a hard material in the seat area.
The seat area is particularly crucial for the reliability of the exhaust valve, as the valve has to close tightly to function correctly. It is well-known that the ability of the seat area to close tightly can be reduced by corrosion in a local area by a so-called burn through, where across the annular sealing surface a channel-shaped gutter emerges, through which hot gas flows when the valve is closed. Under unfortunate circumstances, this failure condition can arise and develop into a rejectable valve during less than 80 hours' operation, which means that often it is not possible to discover the beginning failure at the usual overhaul. Therefore, a burn through in the valve seat may cause unplanned shut-downs. If the engine is a propulsion engine in a ship, the failure may arise during a single voyage between two ports, which may cause problems during the voyage and unintended expensive waiting time in port.
With a view to preventing burn throughs in the valve seat many different valve seat materials with ever increasing hardness have been developed over the years to make the seat wear-resistant by means of the hardness and reduce the formation of dent marks. The dent marks are a condition for development of a burn through as the dents may create a small leak through which hot gas flows. The hot gas can heat the material around the leak to a level of temperature where the gas with the aggressive components has a corrosive effect on the seat material so that the leak rapidly grows larger and the leakage flow of hot gas increases, which escalates the erosion. In addition to the hardness, seat materials have also developed towards a higher hot corrosion resistance to delay erosion after the occurrence of a small leak.
An exhaust valve of the above type and manufactured from the material NIMONIC 80A is described in the article `Herstellung von Ventilspindeln aus einer Nickelbasislegierung fur Schiffsdieselmotoren`, Berg- und Huttenmannische Monatshefte, volume 130, September 1985, No. 9. The thermo-mechanical forging is controlled so that a high hardness is achieved in the seat area. In consideration of the mechanical properties of the exhaust valve, such as fatigue resistance, etc., the article prescribes that the NIMONIC 80A valve has a yield strength of at least 800 MPa.
EP-A-0 280 467 describes an exhaust valve made of NIMONIC 80A manufactured from a base body forged into the desired shape after solution annealing. The seat area is thus cold-worked for provision of high hardness. Subsequently the valve can be precipitation-hardened.
The book `Diesel engine combustion chamber materials for heavy fuel operation` published in 1990 by The Institute of Marine Engineers, London, collects the experience gained for exhaust valve materials in a number of articles and provides recommendations as to how to design valves to achieve long life. Concerning valve seats the articles unanimously direct that the seat material has to have a high hardness and be of a material with a high resistance against hot corrosion. A number of different preferred materials for exhaust valves are described in Paper 7 of the book `The physical and mechanical properties of valve alloys and their use in component evaluation analyses`, including in its analysis of the mechanical properties of the materials a comparative table of the yield strength of the materials, seen to be below about 820 MPa.